The present invention relates to a method of reclaiming various kinds of crosslinked rubbers such as non-sulfur crosslinked rubber, sponge rubber, silicone rubber and diene-type rubber.
Heretofore, a reclaiming method wherein crosslinked rubber is converted into reclaimed rubber by applying heat and shear stress thereto has been known as a method of reutilizing wastes of rubber moldings such as tire wastes, or scrap, defective moldings etc. occurring in a process for producing rubber moldings.
According to the above reclaiming method, cross-linkages among rubber molecules constituting crosslinked rubber are cleaved by heat and shear stress, thus making the rubber molecules in a state similar to non-crosslinked molecules.
By re-crosslinking such reclaimed rubber singly or after being blended with new non-crosslinked rubber, the reclaimed rubber can form moldings.
The crosslinked rubber can thereby be recycled.
However, there are some kinds of crosslinked rubbers whose reclamation is difficult or not feasible by the prior art techniques described above.
For example, even if rubber such as non-sulfur crosslinked rubber (peroxide-crosslinked rubber) is heated and subjected to shear stress, the rubber is merely pulverized by the shear stress, resulting in little cleavage of the cross-linkages. Pulverization of rubber into fine powder is estimated to cause the rubber molecules to be hardly subjected to shear stress.
Further, when sponge rubber having a foamed phase, or a rubber blend prepared by blending such sponge rubber with normal crosslinked rubber (solid rubber), is reclaimed according to the prior art techniques, there arose the following problems.
Also, because of the gaseous components such as entrained air, the sponge rubber is poor in heat conductivity, requires longer preheating, and is slow in the progress of plasticization. Further, the sponge rubber is hardly sheared due to low bulk density, thus making the progress of plasticization slow. Accordingly, reclamation treatment thereof requires a longer time.
Further, due to entrained gaseous components, the pressure during reclamation treatment may be varied, and there may arise troubles such as pulsation, abnormal blowing or blowing back of rubber, so that stable reclamation treatment thereof was difficult.
For example, rubber such as silicone rubber or a rubber blend containing silicone rubber could not be reclaimed by the conventional method.
This is because main chains or cross-linkages of silicone rubber are stable to heat and difficult to be cleaved. Further, in an oxygen atmosphere, its molecules are recombined due to oxide deterioration.
During reclamation, a rubber blend, e.g., consisting of natural rubber and butadiene-type rubber used in automobile tires is deteriorated due to oxidation with air, thus rendering rubber molecules low-molecular by cleavage of their main chains or causing the molecules to be crosslinked with one another. Accordingly, reclamation that is excellent in qualities was difficult in the conventional method.
The present invention was made in view of these problems in the prior art, and the object of the present invention is to provide a method of reclaiming crosslinked rubber, which is capable of reclaiming various kinds of crosslinked rubbers whose reclamation is difficult.
The present invention relates to a method of reclaiming crosslinked rubber including a step of reclaiming crosslinked rubber by applying shear stress to the crosslinked rubber, wherein the maximum pressure in the reclaiming step is 1.5 MPa or more.
The most remarkable feature of the present invention is that the maximum pressure in the reclaiming step is 1.5 MPa or more.
When the maximum pressure is less than 5 MPa, crosslinked rubber may not be reclaimed.
The upper limit of the maximum pressure is preferably 100 MPa. Since a device capable of realizing the maximum pressure of higher than 100 MPa is very large-scale, realization may be difficult.
The lower limit of the maximum pressure is preferably 2 MPa, and the lower limit of the maximum pressure is more preferably 3 MPa.
By increasing the pressure in the reclaiming step as described above, rubber molecules in crosslinked rubber, whose reclamation is difficult in the conventional method because of failure in sufficiently subjecting them to shear stress, can be sufficiently subjected to shear stress, thus efficiently cleaving the cross-linkages therein to reclaim the crosslinked rubber whose reclamation is difficult.
According to the present invention as described above, a method of reclaiming crosslinked rubber can be provided, which is capable of reclaiming various kinds of crosslinked rubbers whose reclamation is difficult.
The reclaiming step in the present invention preferably includes a preheating step and a plasticizing step. In this plasticizing step, shear stress is applied while the maximum pressure is made preferably 1.5 Pa or more.
In the reclaiming method of the present invention, the temperature of crosslinked rubber to be reclaimed is increased due to evolution of heat by shear stress and/or heating from the outside. This is the preheating step.
When the crosslinked rubber attains a predetermined temperature in the preheating step and is further subjected to shear stress, cleavage of cross-linkages and/or some main chains in the crosslinked rubber is initiated, thus softening the crosslinked rubber. This is the plasticizing step.
These steps in the reclaiming method of the present invention can be independently conducted, but from the viewpoint of efficiency of production, it is preferable that the respective steps are conducted successively in one vessel. In this case, the process may proceed successively or simultaneously because the boundary between the respective steps is indefinite.
The two steps described above may be followed by a step of further applying shear stress to the softened crosslinked rubber in order to cleave cross-linkages sufficiently and to disperse and mix rubber molecules. This step is the kneading step. By this step, reclaimed rubber of uniform quality can be obtained.
In addition to the steps described above, there may be further steps such as a step of blending with other materials, a step of re-crosslinking by adding a vulcanizing agent, a step of deodorization, degasification and exhaust, and a step of denaturation-and modification by adding an additive and a reacting agent.
In the reclaiming method of the present invention, the crosslinked rubber is sheared preferably at such a temperature that the cross-linkages in the crosslinked rubber are cleaved and simultaneously cleavage of main chains of rubber molecules constituting the crosslinked rubber does not extremely proceed.
As the shear stress applied is increased, cross-linkages are cleaved more easily, and thus as the shear stress is increased, the temperature during reclamation (temperature of the crosslinked rubber and/or reclaimed rubber) can be lowered.
Specifically, the reclaiming step is conducted preferably at the temperature of 100 to 520xc2x0 C. When the reclaiming step is conducted at a temperature of less than 100xc2x0 C., the cleavage of cross-linkages etc. may not sufficiently proceed. Also, when the temperature is higher than 520xc2x0 C., cleavage of main chains may extremely proceed to deteriorate the physical properties of reclaimed rubber. The upper limit of the above temperature range is more preferably 450xc2x0 C.
In the reclaiming step, the crosslinked rubber is heated or cooled as required so as to be in the above temperature range. When the heat evolved by the crosslinked rubber upon shearing is too low, the rubber is heated, and when the heat evolved is too high, the rubber is cooled down. When the above temperature range is attained by the heat evolved by the crosslinked rubber upon shearing, heat transfer (heating or cooling) from the outside is not necessary.
The optimum temperature range is varied depending on the type of the crosslinked rubber. For example, when automobile tires etc. are to be reclaimed, the temperature range is preferably 180 to 360xc2x0 C. When peroxide-crosslinked rubbers such as EPDM are to be reclaimed, the temperature range is preferably 220 to 450xc2x0 C.
The upper limit of the temperature range during reclamation is varied depending on the duration of reclamation, and the temperature should be raised when short.
The shear stress applied in the plasticizing step in the present invention is preferably 1 to 100 MPa. When the shear stress is less than 1 MPa, cleavage of cross-linkages cannot sufficiently proceed and the efficiency of reclamation may be lowered. When the shear stress is higher than 100 MPa, the shear stress may cause not only cleave of cross-linkages but also extreme cleavage of main chains, whereby the physical properties of the reclaimed rubber may be deteriorated.
The upper limit of the shear stress is more preferably 15 MPa.
The optimum range of the shear stress is also varied depending on the type of crosslinked rubber, and the shear stress is most preferably 1 to 5 MPa for e.g. automobile tires. When the crosslinked rubber is used peroxide-crosslinked rubber such as EPDM, the shear stress is preferably 3 to 10 MPa.
The shear stress can be calculated by multiplying the shear rate in a device used for applying shear stress by the viscosity of the crosslinked rubber.
The crosslinked rubber includes non-sulfur-type crosslinked rubber such as peroxide-crosslinked rubber.
The peroxide-crosslinked rubber is crosslinked rubber crosslinked by a peroxide-based crosslinking agent, and examples thereof include EPDM (ethylene/propylene/diene terpolymer), EPR (ethylene/propylene rubber), NBR (acrylonitrile/butadiene rubber), silicone rubber etc.
When such crosslinked rubber is subjected to shear stress for reclamation, the rubber is merely pulverized by the shear stress, resulting in little cleavage of the cross-linkages. Pulverization of rubber into fine powder is estimated to cause the rubber molecules to be hardly subjected to shear stress.
According to the present invention, such crosslinked rubber or a rubber blend containing such crosslinked rubber can be reclaimed, thus enabling recycling of more kinds of rubber products.
The present invention can also be applied to crosslinked rubbers other than those described above.
Like the conventional method, various reclaiming promoters and additives such as coloring agents, fillers and antioxidants can be added in the reclaiming method of the present invention.
The present invention further provides a method of reclaiming crosslinked rubber including a step of reclaiming crosslinked rubber by applying shear stress to the crosslinked rubber, wherein the crosslinked rubber is sponge rubber and the degree of packing in the reclaiming step is 80 vol-% or more.
The most remarkable feature of this invention is that the degree of packing in the reclaiming step is 80 vol-% or more.
If the degree of packing is less than 80 vol-%, the heat conductivity of the crosslinked rubber may be low, the progress of preheating may be slow, and the efficiency of reclamation may be lowered. Further, loading of the shear stress to the crosslinked rubber proceeding from the preheating step to the plasticizing step would be insufficient to make cleavage of cross-linkages difficult, and thus the efficiency of reclamation may be lowered and reclamation may be insufficient.
The degree of packing is more preferably 95 vol-% or more.
The reclaiming step in the present invention preferably includes a preheating step and a plasticizing step.
In this case, the degree of packing in the plasticizing step is preferably 80 vol-% or more. Further, the plasticizing step is preferably followed by a step of degasification.
The degree of packing is determined by dividing the volume of a solid part of sponge rubber (excluding constrained air) by the volume of a zone for retaining the sponge rubber in the reclaiming step (that is, the preheating step or the plasticizing step).
By raising the degree of packing in the preheating step and plasticizing step as described above, the volume of gas contained in sponge rubber having a foamed phase and in a rubber blend containing such sponge rubber is decreased, and thus the heat conductivity of such material can be increased, and the time required in the preheating step can be shortened.
By raising the degree of packing, the bulk density of sponge rubber can be increased, and the shear stress can be efficiently applied to the sponge rubber. Accordingly, the efficiency of reclamation can be increased, thus realizing shortening of the time needed for reclamation.
In the present invention, there is the step of degasification. The step of degasification is a step where gaseous components are removed from the place where the reclamation treatment is conducted.
Accordingly, a rapid fluctuation in the pressure during the reclamation treatment can be prevented, and troubles such as pulsation, abnormal blowing and blowing back of rubber can be prevented. Therefore the reclamation can be carried out safely and stably. Accordingly, the method of reclaiming crosslinked rubber can be provided, which is capable of reclaiming a wide variety of crosslinked rubbers whose reclamation is difficult.
In this reclaiming method, the maximum pressure in the plasticizing step is preferably 1.5 MPa or more as described above.
With this arrangement, the shear stress and optimum temperature necessary for reclaiming sponge rubber can be efficiently applied.
The upper limit of the maximum pressure is preferably 100 MPa. Since a device capable of realizing a pressure of higher than 100 MPa is very large-scale, realization may be difficult.
The lower limit of the maximum pressure is preferably 2 MPa. The lower limit of the maximum pressure is more preferably 3 MPa.
The sponge rubber described above includes sponge rubber having an expansion ratio of 1.5 or more and a specific gravity of 0.75 or less.
Such sponge rubber is hardly reclaimed by the conventional reclaiming method by shear stress, but effectively reclaimed according to the reclaiming method of the present invention.
For a rubber blend prepared by blending the sponge rubber with other rubber, the same effect can also be obtained.
This reclaiming method can be applied to sponge rubber such as sulfur-crosslinked (vulcanized) EPDM composed of various materials.
For the reclamation in this invention, the temperature is preferably in the range of 100 to 520xc2x0 C., which is the same as in another present invention described above. The shear stress in the plasticizing step is preferably in the range of 1 to 100 MPa, which is also the same as in another present invention described above. The upper limit of the shear stress is more preferably 15 MPa.
This invention, similar to another present invention described above, also includes a preheating step and a simultaneously with a step after the plasticizing step (see Embodiment 3).
The degasifying step is performed by using e.g. an extruder provided with vent(s) arranged just over each steps such as plasticizing step and a full-flighted screw for connecting the steps such as plasticizing step.
Other features are the same as in the reclaiming method described above.
The reclaiming method is preferably conducted by use of an extruder that is pressurized by a means of suppressing the forward conveying of crosslinked rubber to an extrusion orifice of the extruder.
By suppressing the feeding of crosslinked rubber, the degree of packing of crosslinked rubber in the cylinder can be easily increased.
The feeding of crosslinked rubber in the present invention is suppressed but not stopped, so the crosslinked rubber (turning into the reclaimed rubber as the reclamation process proceeds) is sent gradually toward the extrusion orifice.
The suppressing means is preferably constituted by the screw possessed by the extruder, wherein the direction of the screw is switched at a certain position in the extruder (see FIG. 1). The feeding of crosslinked rubber can thereby be easily realized.
The extruder used is e.g. a twin-screw extruder having a cylinder with an extrusion orifice and a screw arranged in the cylinder, as shown in FIG. 1 described below.
In the case where such twin-screw extruder is used, the crosslinked rubber is introduced into the cylinder, heated in the cylinder by an external heating source, and subjected to shear stress by rotation of the screw.
The direction of the screw can be switched at a switching part in the screw as shown in FIG. 1. By rotating this screw in the cylinder, the crosslinked rubber is loaded, at the switching part, force P1 in the direction toward the extrusion orifice and force P2 in the opposite direction, so that forward conveying of the crosslinked rubber is suppressed and stopped at the switching part, thereby pressurizing the cylinder. The degree of packing of the crosslinked rubber in the cylinder can be raised by suppressing forward conveying of the crosslinked rubber and stopping the forward conveying.
The present invention also provides a method of reclaiming crosslinked rubber including a step of reclaiming crosslinked rubber by applying shear stress to the crosslinked rubber, wherein a main chain-cleaving agent is present in the reclaiming step. Crosslinked rubber such as non-sulfur-type crosslinked silicone rubber whose reclamation is difficult by the conventional reclaiming method and whose qualities are hardly influenced even if its main chains are cleaved can be reclaimed according to this method.
According to the present invention as described above, a method of reclaiming crosslinked rubber, which is capable of reclaiming various kinds of crosslinked rubbers such as silicone rubber whose reclamation is difficult, can be provided.
As the main chain-cleaving agent, a material capable of cleaving siloxane bonds is preferably used. By cleaving siloxane bonds constituting non-sulfur-type crosslinked silicone rubber, reclamation of the silicone rubber can be realized.
The material includes e.g. basic catalysts such as potassium hydroxide, polar solvents such as dimethyl formamide and dimeithyl sulfoxide, acid catalysts such as hydrochloric acid and active china clay, water, and alcohols such as ethanol, butanol and isopropyl alcohol.
It is preferable that the reclaiming step includes a preheating step and a plasticizing step, and the main chain-cleaving agent is added during the plasticizing step. With this arrangement, the main chain-cleaving agent can effectively act on the crosslinked rubber.
When the main chain-cleaving agent is added during the process, a pressure pump is preferably used. Alternatively, the main chain-cleaving agent may be allowed to be coexistent with crosslinked rubber by previously impregnating the crosslinked rubber therewith.
This reclaiming method also includes a preheating step and a plasticizing step, which is similar to another reclaiming method described above, and these steps are the same as described above.
The main chain-cleaving agent is added preferably in an amount of 0.1 to 20 parts by weight based on 100 parts by weight of the crosslinked rubber. With this arrangement, main chains in silicone rubber etc. can be certainly cleaved to realize reclamation of the crosslinked rubber. If the amount of the added main chain-cleaving agent is less than 0.1 part by weight, cleavage of main chains in the crosslinked rubber may be insufficient to make reclamation of the crosslinked rubber difficult, while if the amount exceeds 20 parts by weight, the characteristics of reclaimed rubber cannot further be improved, the ability to reclaim the rubber maybe lowered, and the material characteristics of the resulting reclaimed rubber can be deteriorated due to the presence of the non-acting main chain-cleaving agent.
The lower limit is preferably 0.5 part by weight, more preferably 5 parts by weight.
The above reclaiming step is conducted preferably at 100 to 520xc2x0 C. as described above.
The present invention also provides a method of reclaiming crosslinked rubber including a step of reclaiming crosslinked rubber by applying shear stress to the crosslinked rubber, wherein the crosslinked rubber is pressurized in the reclaiming step, and the reclaiming step is conducted in a non-oxidizing atmosphere.
By using the non-oxidizing atmosphere, the reclaimed rubber can be prevented from being oxidatively deteriorated by air and oxygen gas, thus preventing re-crosslinking of the molecules during reclamation from occurring and improving the quality of the reclaimed rubber.
By pressurization, rubber molecules in crosslinked rubber, whose reclamation is difficult by the conventional method because of failure in sufficient subjection of shear stress, can be sufficiently subjected to shear stress, thus efficiently cleaving the cross-linkages therein to reclaim the crosslinked rubber whose reclamation is difficult.
The same preheating and plasticizing steps, as described above, are also preferably conducted in this reclaiming method.
As described above, the reclaiming step is conducted preferably at the temperature of 100 to 520xc2x0 C.
The crosslinked rubber which is easily oxidatively deteriorated includes diene and butadiene type rubber such as SBR (styrene-butadiene rubber), NBR (acrylonitrile-butadiene rubber) and BR (butadiene rubber), as well as rubber blends including such diene or butadiene type rubber and other rubber.
The means of realizing the non-oxidizing atmosphere includes a method of replacing the air in ambient atmosphere by non-oxidizing gas when the crosslinked rubber is introduced into a reclaiming device (e.g. twin-screw extruder in FIG. 4), whereby the air is introduced together with the crosslinked rubber into the device.
As the non-oxidizing atmosphere, a nitrogen gas atmosphere etc. can be used.
The reclaimed rubber obtained in this reclaiming method can be reutilized as rubber moldings by re-crosslinking it with a crosslinking agent. Further, the reclaimed rubber can also be reutilized as rubber moldings by mixing new non-crosslinked rubber with the reclaimed rubber and re-crosslinking the resulting blend with a crosslinking agent.
The reclaimed rubber obtained in this reclaiming method is in such a state that gel components insoluble in toluene (or good solvent for the reclaimed rubber) remain in an amount of preferably 20% by weight or more, more preferably 30% by weight or more, and most preferably 40% by weight or more.
When the amount of gel components insoluble in toluene is less than 20% by weight, cleavage of not only cross-linkages but also of main chains of rubber molecules may proceed extremely. The reclaimed rubber is often sticky, and the physical properties and processability thereof may be lowered.
The amount of gel components is preferably less than 90% by weight, more preferably 80% by weight and most preferably 70% by weight. When the amount is 90% by weight or more, reclamation of the crosslinked rubber is estimated to be insufficient. Accordingly, dispersibility of the resulting reclaimed rubber in new non-crosslinked rubber and adhesion thereof may be lowered, and the surface properties and mechanical properties of rubber moldings obtained by re-crosslinking the reclaimed rubber may be deteriorated.
Although the gel components described above are varied depending on the type of reclaimed crosslinked rubber, for example, they are made of e.g. polymer gel formed by three-dimensional crosslinkage of rubber, carbon gel including rubber and carbon black, and inorganic materials such as carbon black.
The network chain density of rubber in gel components in the reclaimed rubber obtained by this reclaiming method is preferably in the range of {fraction (1/50)} to xc2xc relative to that of the crosslinked rubber before reclamation. Such reclaimed rubber is in such a state that its rubber molecules maintain a crosslinked structure to a certain degree.
When the network chain density of rubber in the gel components exceeds xc2xc relative to the network chain density in the crosslinked rubber before reclamation, the dispersibility thereof in new non-crosslinked rubber and compatibility thereof with the new non-crosslinked rubber may be deteriorated because of insufficient reclamation of the crosslinked rubber.
On the other hand, when the network chain density therein is less than {fraction (1/50)}, cleavage of not only the cross-linkages but also of main chains of rubber molecules proceeds, thus readily making the reclaimed rubber sticky and deteriorating the physical properties and processability thereof.
The network chain density therein is more preferably in the range of {fraction (1/20)} to xc2xc most preferably {fraction (1/20)} to xe2x85x9.
The Mooney viscosity (ML1+4, 100xc2x0 C.) at 100xc2x0 C. of the reclaimed rubber obtained in this reclaiming method is preferably 10 to 150. When the Mooney viscosity exceeds 150, the dispersibility thereof in new non-crosslinked rubber and compatibility thereof with the new non-crosslinked rubber may be deteriorated because of insufficient reclamation of the crosslinked rubber. When the Mooney viscosity is less than 10, cleavage of main chains of rubber molecules proceeds, thus readily making the reclaimed rubber sticky and deteriorating the physical properties thereof.
The Mooney viscosity is more preferably in the range of 15 to 120, most preferably in the range of 20 to 80.